Looking for an interesting RTS that is not only free, but also open-source?
Well look no further than Wesnoth!
The Battle for Wesnoth, or simply Wesnoth, is a turn-based strategy game with a fantasy setting, designed by David White and first released in June 2003. In Wesnoth, the player attempts to build a powerful army by controlling villages and defeating enemies for experience. White based Wesnoth loosely on the Sega Genesis games Master of Monsters and Warsong. He wanted to create a freely-available, open source strategy game with very simple rules, but one that had strong artificial intelligence and that was challenging and fun.
The Battle for Wesnoth is available under the GPL license in source form and for a variety of computer operating systems.